A double overhead camshaft internal combustion engine has intake and exhaust camshafts mounted on a cylinder head. The intake and exhaust camshafts are operatively connected to each other with helical gears and are timely turned by means of a timing chain stretched between a sprocket wheel secured to either one of the intake and exhaust camshafts and a sprocket wheel secured to an output shaft, such as a crankshaft, of the double overhead camshaft internal combustion engine.
To prevent the intake and exhaust camshafts from causing thrust movement due to the operative connection of the intake and exhaust camshafts with the helical gears, each of the intake and exhaust camshafts is generally provided with a thrust restrictive collar which is received in an annular groove which is formed in a thrust bearing block which is divided into two halves: an upper half being formed integrally with the cylinder head of the engine body and a lower half being formed integrally with a bearing cap and hereinafter referred to as a plain split thrust bearing block.
In the meantime, as a lubrication system for the thrust restrictive structure for the intake and exhaust cam shafts of the double overhead camshaft engine, it is popular to lubricate the thrust restrictive collars in the annular grooves formed in the plain split thrust bearing blocks by lubrication oil supplied from outside the thrust restrictive structures. In such a lubrication system, however, an external oil passage or passages has or have to be provided to carry the lubrication oil to the thrust collars. In an attempt at eliminating such an arrangement of oil passages, the overhead camshaft is formed with an oil passage extending along its length in parallel with an axial centerline of rotation thereof; the axial oil passage is communicated with an internal oil passage, radially extending from the axial oil passage formed and opening to the annular groove of the plain split thrust bearing block, formed in the thrust collar to carry the lubrication oil to the thrust collar. Forming the radial oil passage causes a decrease in structural rigidity or stiffness of the thrust collar.
To understand physical phenomena occurring in the overhead camshafts formed with the thrust collars having radial oil passages penetrating therethrough for, in particular, three cylinder internal combustion engines, such as in-line three cylinder internal combustion engines or V-6 type internal combustion engines, reference is had to FIGS. 1(A) to 1(C). Because of such an overhead camshaft having three, or three pairs of, cams for three cylinders which are adapted not to overlap in lift cycles relative to one another, driving torque for the intake and exhaust camshafts varies considerably with changes of crank angle or valve lift as shown by graphs in FIGS. 1(A) and 1(B), respectively. Because a peak of change in driving torque in an exhaust cycle for each cylinder is substantially the same in time-phase as a peak of change in driving torque in an intake cycle for a cylinder fired ahead of the cylinder, the interconnected intake and exhaust camshafts interact with each other to cause an increase in torque change developed by each overhead camshaft as shown by a graph in FIG. 1(C). Such a torque change is altered or changed to a thrust loaded on the overhead camshaft through the helical gears. Accordingly, the provision of radial oil passages opening to the annular groove of the plain split thrust bearing block leads to a serious problem of a decrease of rigidity of the thrust collar.
The plain split thrust bearing block for supporting the overhead camshaft, which generally consists of upper and lower halves bolted to each other, is formed with an inner groove divided into upper and lower halves for receiving therein the thrust collar. Because of the difficulty of accurately aligning upper and lower inner grooves of the upper and lower halves of the plain split thrust bearing block, either one of the upper and lower inner grooves is formed wider than the other inner groove so as to substantially support the thrust collar by the one of the upper and lower halves of the plain split thrust bearing block only. In the case of using such a plain split thrust bearing block together with the thrust collar with oil passages opening to the inner groove of the plain split thrust bearing block, the thrust collar is considerably lowered in rigidity or strength against thrust when the radial oil passage is located by the thrust collar in parallel with a fitting surface of the upper and lower halves of the plain split thrust bearing block, particularly, at a moment a maximum thrust is loaded on the overhead camshaft.